<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }

    .content {
      overflow: hidden;
      width: 1000px;
      height: 3000px;
      background-color: pink;
      margin: 0 auto;
    }

    .backtop {
      display: none;
      width: 50px;
      left: 50%;
      margin: 0 0 0 505px;
      position: fixed;
      bottom: 60px;
      z-index: 100;
    }

    .backtop a {
      height: 50px;
      width: 50px;
      background: url(./images/bg2.png) 0 -600px no-repeat;
      opacity: 0.35;
      overflow: hidden;
      display: block;
      text-indent: -999em;
      cursor: pointer;
    }

    .header {
      position: fixed;
      top: -80px;
      left: 0;
      width: 100%;
      height: 80px;
      background-color: purple;
      text-align: center;
      color: #fff;
      line-height: 80px;
      font-size: 30px;
      transition: all .3s;
    }

    .sk {
      width: 300px;
      height: 300px;
      background-color: skyblue;
      margin-top: 500px;
    }
  </style>
</head>

<body>
  <div class="header">我是顶部导航栏</div>
  <div class="content">
    <div class="sk">秒杀模块</div>
  </div>
  <div class="backtop">
    <img src="./images/close2.png" alt="">
    <a href="javascript:;"></a>
  </div>
  <script>
    const sk = document.querySelector('.sk')
    const header = document.querySelector('.header')
    window.addEventListener('scroll', function () {
      console.log(sk.offsetTop);
      console.log(document.documentElement.scrollTop);
      if (document.documentElement.scrollTop >= sk.offsetTop) {
        header.style.top = 0
      } else {
        header.style.top = '-80px'
      }
    })

  </script>
</body>

</html>